Plural beam electron gun



Sept 28, 1954 F'. H. NlcolLL ET AL 2,690,517

PLURAL BEAM ELECTRON GUN Filed Sept. 50, 1952 ORNEY Patented Sept. 28, 1954 riso PLURAL BEAM ELECTRON GUN Application September 30, 1952, Serial No. 312,298

(Cl. S13- 70) 11 Claims. l

This invention is directed to a cathode ray tube and more particularly to the gun structure for a television picture tube.

In many television applications it is desirable that the brightness of the picture on the fluorescent screen be as great as is possible. This is particularly true in television projection type picture tubes in which the image from the iuores cent screen of the tube is projected onto a viewing screen. Also, some picture tubes used to produce television pictures in color require a plu rality of electron beams.

Direct View color tubes of the type employing direction sensitive screens require electron guns producing two or more beams of electrons ap preaching the screen at angles, such that discrete colors will be produced by each beam. In

particular, one form of tri-color tube employsl beams arranged 120D apart around the tube and approaching the screen at an angle of about 1.2 to the normal to the screen.

These three beams may be obtained by physically pointing three separate guns, or by the use of three separate and parallel guns, the electron beams being converged by an electrostatic lens to form three focused beams at the screen. l

Inherent inaccuracies in mechanical construem tion of the guns make it very diiicult to direct multiple beams With the accuracy required, and as a result small magnets are required to position the individual beams and produce the desired relative location of the beams. In addition, the three lbeams then have to be moved as a group by an auxiliary def-looting means so that each of the three beams will pass through the correct point near the center of the ordinary deflection yoke. This is necessary in order to obtain pure color fields with each beam.

Another diiculty with the present gun is the complication and cost of three complete assemblies and the fact that the three gun assemblz. occupies a large space. The achievement of smaller angles of convergence isalso limited by the physical size of each gun.

It is an object of the invention to provide a novel gun structure for producing a plurality oi electron beams.

It is a further object of the invention to provide a television picture tube having an improved gun structure for producing a pluralty or" electron beams, which are caused to converge at a com mon point on the tube screen.

It is a further object of the invention to pro vide a simplified electron gun structure for pro f ducing a multiple number of beams which are brought to a common point independent of inaccuracies in mechanical construction, thus providing relative ease of adjustment.

A further object of the invention is to provide a novel electron gun structure producing a multiple number of beams by the use o a simple and relatively inexpensive gun assembly.

It is a further object of the invention to pron vide a novel electron gun structure for a tela vision picture tube producing a multiple number of electron beams and in which the gun structure occupies a relatively small space,

The invention is directed to an electron gun structure for a television type picture tube with which a plurality of electron beams are formed each from a separate cathode. The plurality cathodes and the apertured-electrodes used therewith are arranged so that all of the bea-ms appear to come from a single point. These when focused to their smallest diameter on the fluorescent screen by a focusing electron field must, of necessity, converge to a point at the screen. The specific gun structure comprises :a plurality of cathode elements mounted Within a common control grid structure having a first diaphragm with an aperture positioned over the emitting surface of each cathode, and in which the control grid structure also has a diatxi-phragm with a single aperture on an axis disposed equi-distant from the cathodes. The grid structure in cooperation with an anode electrode provides an electron lens field which causes the emission from the cathodes to converge with a .minimum cross-sectional area at a common point. The electron beams diverging from the common point are then simultaneously focused and converged on the fluorescent screen. This process is essentially one of imaging the common point by the electron lens on the screen.

Fig. 1 is a sectional View of a cathode ray tube using a novel electron gun structure in accordance with the invention.

Fig. 2 is an enlarged detail drawing oi the 'gun structure of Fig. 1.

Fig. 3 is a sectional View of another form electron gun structure in accordance with the invention.,

Fig. 4 is a cross-sectional view o the structure shown in Fig. 2 along the plane of the line In Fig. l there is disclosed a cathode ray tube utilizing a' plurality of electron beams. The

tube comprises an evacuated envelope iii having a large shellV portion l2 and a tubular neck portion ll. Mounted within the neck portion Eil i:

an electron gun structure |16 consisting essentially of a plurality of cathode electrodes I8 for forming a multiple number of electron beams directed at a target electrode I5.

As shown in greater detail in Fig. 2, there are at least two cathodes IB mounted symmetrically with respect to the axis 20 of the tubular neck portion I4. Cathodes I8 are essentially thin walled metal tubes having the end facing the bulb portion I2 closed respectively by wall portions, the surfaces of which are coated with any Well-known electron emitting material such as a mixture of barium and strontium oxides. Heater filaments 24 are respectively mounted within the tubular cathodes I8 to maintain the cathode surfaces at a suitable temperature during tube operation.

Cathodes I8 are fixed in a parallel space arrangement through a common insulating ceramic disk 26 which is, in turn, mounted within a tubular control grid electrode 28. Closely spaced from the coated surfaces of the electrodes I8 is a portion of the control grid 28 consisting of an apertured diaphragm or plate 30. An aperture 32 of plate 30 is positioned over the electron emitting surface of each cathode I8. Closely spaced within the tubular control electrode 23 and from the apertured plate 30 is a second aperturecl plate or diaphragm 34 having a single aperture 36 positioned coaxially on the axis '20. The tubular control grid 28 is closed 'by a third element 38, also having a single aperture 40 formed coaxially on axis 20. There is positioned between control electrode 28 and the fluorescent screen I of Fig. 1 a tubular accelerating electrode 42 (not shown in Fig. 1). The conical envelope portion I2 may be either formed of glass or of a metal shell, as shown in Fig. 1. A conductive coating 44 extends from a point adjacent the end of the tubular electrode 42 facing target I5 over the inner surface of the tubular envelope portion I4 toward the fluorescent screen. Coating 44 is extended to make contact with the metal envelope shell portion I2. During tube operation, the metal shell, the conductive coating 44 and the tubular electrode 42 are maintained at a common potential. For this purpose, the metal shell I2 may be connected to an external source of potential by a lead means indicated at 4G. Tubular electrode 42 is connected externally to the same source of potential through conductive coating 44 and spring fingers 48 fixed to the upper portion of electrode 42 by means of a support plate 50, wall coating 44 being, in turn, connected to the external voltage source through the metal shell I2 and lead 46 as shown.

The fluorescent screen I5 may be of any desired form, such as merely a single lm of white luminescing phosphor if the tube is to be used as a television viewing tube or a projection tube. Also, the fluorescent screen structure may be one of several types used in a multi-color television viewing tube. Such a screen strutcure may be similar to those described in the U. S.. Patent 2,581,487 to D. A. Jenny. The specific screen in Fig. 1 consists principally of a supporting transparent plate 52 on the surface of which is formed a fluorescent layer 54 in the form of spaced dots, in which adjacent dots iiuoresce with different primary colors. Closely spaced adjacent the fiuorescent layer 54 and on the side of screen I5 facing the electron gun I6 is an apertured masking electrode 56 having a multiple number of apertures 58 therethrough. Electron gun I6 forms a plurality of electron beams which are focused and brought to convergence on the fluorescent layer 54. If the beams converge from different angles, they will pass through the apertures 53 of the masking electrode 56 and will strike the fiuorescent screen 54 at distinctly different spots. The fluorescent material is deposited on the supporting plate 52 in an arrangement whereby each beam passing through any of the apertures 58 of the masking electrode 56 will strike only phosphor spots iiuorescing in a single color.

In accordance with the invention, the control grid electrode 28 is operated at ground potential while the accelerating electrode 42 is operated at a high positive potential in the order of 10,000 volts. As shown specifically in Fig. 2, the cathode electrodes are both connected respectively through a resistance 50 to a voltage divider 52 for varying the potentials of cathodes I8 respectively between ground and a positive 60 volts relative to ground.

During tube operation with the given potentials applied respectively to electrodes 28 and 32, there is produced an intense electrostatic field between electrode 42 and the control electrode 28. The apertures 40' and 36 of plates 38 and 34, respectively, are su'iliciently large that the positive field of electrode 42 extends through these plates 38 and 34 into the tubular control electrode 28. This field is only partially indicated by the lines 64, which represent the field formed by equi-potential surfaces extending through the aperture 40 and bulging outwardly to form curved surfaces symmetrically disposed about the common gun axis 20. Furthermore, this field extends through plate 34 and through apertures 32 in the control electrode plate 30. The iield portions extending through apertures 32 form a small convergent electron lens within each aperture 32.

Electrons emitted by the positive cathodes I3 are urged by the accelerating field G4 (formed by the equi-potential surfaces) through the grounded control electrode plates 35, 34 and 38. Due to the arrangement of the respective apertures 32, 36 and 40, the accelerating field formed as described above causes the electrons from each cathode to be formed into beams 6G respectively. The electron beams 6B enter the accelerating field off the axis 25 and thus pass through the equi-potential surfaces t4 of the field at an angle. There is a component of force which urges the electrons in the direction of the field and thus deflects the beams toward axis 20 and in a manner such that the beams will converge and cross each other at a common point B8. Furthermore, by the choice of the electrode dimensions and independent of the strength of the eld used, the electrons of each beam will also be focused or converged to a minimum crosssectional area at the common point 68 of convergence.

To assure symmetry of beams 66 about the axis 20, it is necessary that the apertures 32 in the control electrode plate 30 be accurately positioned symmetrically to or equidistant from the lens axis 20. Also, it is necessary that the common axis of symmetry 20 of apertures 32 pass through the center of aperture 40. Furthermore, the axis of symmetry 2l! should also pass through the center of aperture 35 of plate 34, as any positioning of aperture 35 off of axis E@ will distort the field 64 and result in the beams crossing at some other point other than on the axis 20.

The voltages given above for the operating potentials of electrodes 28 and 42 are those which have been used successfully ina tube of the type described. In this tube, the cathodes i8 were nickel cylinders having a diameter of 40 mils. The control electrode apertures 32 were respectively 30 mils each in diameter. Apertures corresponding to apertures 3e and 40 were of diameters 13G and .1.70 mils respectively. The spacing between the apertured portions of plates 30 and 34 was 15 mils and between the apertured portions of plates 3H and 33 was 115 mils. The accelerating electrode i2 was partially closed at its end facing the cathodes i8 with a centrally apertured plate, as shown in Fig. 2, having an aperture diameter of SL70 mils. Also, electrode 42 was spaced from the apertured -control electrode plate 33 by a distance of 100 mils. Voltages were applied in the range of 5 to 9.0 kilovolts to the accelerating electrode d2.

Positioned along the tube axis 2!! and coaxially thereto is a focusing coil t9 (Fig. 1). Current passing through coil 69 produces a symmetrically disposed magnetic field about the axis 2Q. The electron beams 65 diverging from point 68 will enter the field of coil 69 at equal distances from the axis 2b. The action of the field of -coil 69 is to converge the beams to a common point on axis 2@ of the field. The strength of the eld is varied until the convergence point of beams 6B is on the uorescent screen I 5. The action is substantially that in which the converging field of coil 63 images the common point 53 on the screen I5. Furthermore, since the crossover point 68 is also the point of minimum beam cross section, the image of point t8 on screen I6 will also be a minimum. rihus, not only does the iield of coil 69 converge beams @S onto the screen l5, but because the individual beams have a minimum cross section at point Ell, the electrons of each beam are focussed when converged.

The opening of aperture 40 of plate 38 determines the shape of the i'leld B4. A smaller opening would produce a greater curvature of the equi-potential surfaces of held 54 and thus provide a greater axial deflection of beams Et, which will then converge to a cross-over point somewhere in front of point 68 and in a region where the electron beams do not have a minimum crosssectional area. If this new cross-over point were then imaged on screen l5, the beams would not be in focus since the minimum cross-sectional portion of each beam would then be imaged at the screen i5.

Also, conversely, forming the aperture 40 with a larger opening would provide a field Eid having less bulging or curvature of the field surface so that there would be less axial deiiection of the electron beams and the cross-over point 53 would be closer to the fluorescent screen i5 and not at the same point as would be the minimum cross sections of beams (it.

Furthermore, iield 6e is determined by the spacing between the apertured grid plate 38 and the apertured grid plate 3b. This spacing is quite critical, since with the dimensions given, the shape of field 6d is provided with suicient curvature or lens effect to converge the beams to the point E8 at which each beam has its minimum cross-sectional areas. If the spacing between apertured plates 39 and 3B were less, the lens .or convergence effect of the eld 54 would also cause a crossing over of beams EB at a point before they have reached their minimum cross-sectional area. A larger spacing of plates 30 and 38 would pro- 6 l vide a field having less curvature, and hence there would result less convergence of the beams and they would cross over each other at a point beyond point 58 Where the beams form their mini mum cross-sectional area.

The potential difference between the control electrode 28 and the accelerating electrode li? determines the strength of the field 6G. However, the convergence or beams 5K5 is not determined by the iield strength, but only by the shape of the iield. Increasing the potential diierence between electrodes 2t and ft2 does cause the beam Et to form a small cross-sectional area at the cross over point ii resulting in focusing to a sharper point on the uorescent screen. Increasing the potential difference also increases the emission current and cathode voltage required to cut off the beams. The apertured grid plate 313 does not greatly affect the focusing and converging of the electron beams h6 and the same results may be produced with minor Changes in the electrode structure when plate Sli is eliminated.

The focusing coil 69 used with a tube of the type described, and which has been successfully operated, is a standard focusing coil as used in commercial black and white television tubes requiring about 1000 ampere turns for focusing at 2G kilovolts. The magnetic lheld set up under operating conditions is sufiicient to converge the beams 5S on the target i5. If no current were owing through coil 538, the beams ii would continue to diverge from point 68 and strike the fluorescent screen in Widely separated uniocused spots, one for each beam. AS the strength of the eld of coil 5E is increased, these spots are brought closer and closer together and, furthermore, the electrons in each beam are brought closer and closer to focus so that the spots become smaller. At optimum convergence, when all the spots converge to form single common point on the uorescent screen, the electrons in each beam also have optimum focus so that the common point of convergence is also one of focus for each beam. This optimum condition is only possible as mentioned above when the beams have a minimum cross-sectional area at the cross over point tu.

The plural beams coming into the common point or convergence at screen it will approach the screen at very small angles in the order of 1 to 2. The actual angle of convergence is determined by the ratio of the distance from the coil to the cathode and screen, respectively. The plural beams may be used either to intensify the picture or a conventionai picture tube or .may be used for any desired purpose as, for example, in a color television picture tube of the type described.

A beam deflection yoke i@ is normally formed of two pairs of coils, with one coil of each pair disposed on the opposite side oi the tubular neck i4 from the coil of the other pair. The coils of each pair are connected together in series to current sources which provide magnetic elds for respectively providing line and frame scansion of the plural beams over the target i5. The fields produced by each pair of deflecting coils are perpendicular to each other to provide a rectangular type of raster. However, the scanning of the plural beams 66, as well as this invention is not limited to any particular type of beam scansion. The scanning of electron beams over a target surface is well known and is not further described.

Fig. 4 describes a second modification in the invention and consists of plural cathodes 12 mounted in a tubular control electrode 'I4 having a wall or plate portion 'i6 mounted between the cathode 'l2 and the fluorescent screen, not shown. The tubular control electrode M has also a second plate 'i8 mounted beyond plate 7B. Spaced along the axis of the electron gun of Fig. 4 is a short tubular screen electrode 80 partially closed at its end adjacent the cathodes 72 by an apertured plate 82. Spaced from the control grid B and along the gun axis toward the fluorescent screen is an accelerating electrode 811 corresponding to electrode i2 of the structure of Figs. 1 and 2. The plate i6 of the control electrode 'E6 has apertures l5 therethrough with one aperture overlying the electron emitting surface of each cathode 72. The screen grid electrode 8] corresponds to the apertured plate 33 of the gun of Fig. 2. Control electrode la is operated at ground potential and screen grid 80 is operated at a relatively low positive potential relative to ground. The potential of screen grid 3U should not be more than (9,0 of the potential or" the accelerating electrode 84 for optimum operation. As in the case of the structure of Fig. 2, the size of the aperture 83 of screen electrode 8B, as Well as its spacing from the control electrode plate T16, determines the shape of the eld formed between the accelerating electrode il@ and the control grid plate lli. The optimum dimensions or" a tube made similar to the structure described for Fig. 4 was one in which aperture 83 was 170- mils, the aperture in plate la was 170 mils, and each aperture i5 in the control electrode plate 'I5 and the screen electrode plate 82 was 30 mils.

Tubes made with dimensions in the order of those listed above gave optimum tube operation for the voltages indicated. In the manner similar to that described with the structure of Figs. 1 and 2, the electron beams formed from cathodes i2 are respectively brought to a cross over point at which each beam has a minimum crosssectional area. This cross over point is imaged on the fluorescent screen i5 by a converging and focusing field. In the modification of Figs. 1 and 2 such a converging field was described as that formed by a focusing coil 68. However, such a converging and focusing eld may also be produced electrostatically, such as shown in Fig. Li, in which a different potential is put on the coating dll to produce an electrostatic field between the accelerating electrode 845 and coating M. As shown in Fig. 4, the spring fingers 138 of the modification of 1Eig. 2 have been elimnated and accelerating electrode 34 is insulated or spaced from the wall coating lid.

If the tubes of Figs. l, 2 and 4 are to be respectively used for producing color pictures, for example, the incoming color video signals may be applied to the cathode electrodes of the tube gun with each cathode having its potential modulated by a single color signal. In this manner the instantaneous potential of each cathode will be varied by the video signal voltage relative to the grounded control grid 28 and 'I4 respectively. Thus, Fig. 2 shows each cathode l connected through a condenser and respectively to a different source of incoming signals. If the tubes disclosed in the application, however, are to be used with black and white television reception, the cathodes all may be connected to a common source of video signals so 8 that they may be modulated simultaneously with the same signals.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. An electron discharge device comprising, means for producing a plurality of electron beams along respective paths having a common general direction, a target electrode mounted transversely to said beam paths, said beam producing means including electron emitting means, a control 'grid electrode and an accelerating electrode successively spaced between said electron emitting means and said target, said control grid electrode including a iirst grid plate adjacent to and closely spaced from said electron emitting means and having a plurality of apertures therethrough, said control grid electrode including a second grid plate having a single aperture therethrough and positioned between said rst grid plate and said accelerating electrode.

2. An electron discharge device comprising, means for producing a plurality of electron beams along respective paths having a common general direction, a target electrode mounted transversely to said beam paths, said beam producing means including electron emitting means, a control grid electrode and an accelerating electrode successively spaced between said electron emitting means and said target, said control grid electrode including a iirst grid plate adjacent to and closely spaced from said electron emitting means and having a plurality of apertures therethrough symmetrically arranged about a common axis, said cathode grid electrode including a second grid plate positioned between said iirst grid plate and said accelerating electrode and having a single aperture therethrough on said common axis.

3. An electron discharge device comprising,

u electrode means for producing a plurality of electron beams along respective paths having a common general direction, a target electrode mounted transversely to said beam paths, said beam producing electrode means including a plurality of cathode electrodes closely spaced symmetrically about an axis, a single control electrode and an accelerating electrode successively spaced along said axis between said cathode electrodes and said target, said control electrode including a first grid plate adjacent to said cathode electrode and having a plurality of apertures therethrough with one aperture aligned with each cathode electrode and a second grid plate positioned between said first grid plate and said accelerating electrode and having a single aperture therethrough on said axis.

a. An electron discharge device comprising, electron gun means for producing a plurality of electron beams along respective paths having a common general direction, a target electrode mounted transversely to said beam paths, said electron gun means comprising a single rst focussing means for bringing the electrons of said beams each to a minimum cross section at a common point, and a second focussing means between said target and said first focussing means for imaging said common point on said target surface, said rst focussing means including electron emitting means and a rst control grid plate having a plurality of apertures therethrough ad- 9 jacent said electron emitting means and a second control grid plate having a single aperture therethrough between said first grid plate and said target electrode, and means electrically connecting said rst and second grid plates.

5. An electron discharge device comprising,

electron gun means for producing a plurality of electron beams along respective paths having a common general direction, a target electrode mounted transversely to said beam paths, said electron gun means comprising a single rst focussing means for bringing the electrons of said beams each to a minimum cross section at a common point on an axis between said gun means and said target, said electron gun means including a plurality of cathode electrodes closely spaced symmetrically about said axis, a single control electrode adjacent said cathode electrodes and an accelerating electrode successively spaced along said axis from said cathode electrodes, and a second focussing means between said target and said rst focussing means for imaging said common point on said target surface.

6. An electron discharge device comprising, electron gun means for producing a plurality of electron beams along respective paths having a common general direction, a target electrode mounted transversely to said beam paths, said electron gun means comprising a single rst focussing means for bringing the electrons of said beams each to a minimum cross section at a common point on an axis between said gun means and said target, said electron gun means including a plurality of cathode electrodes closely spaced symmetrically about said axis, a single control electrode adjacent said cathode electrodes and an accelerating electrode successively spaced along said axis from said cathode electrodes, said control electrode including a first grid plate having a plurality of apertures therethrough with one aperture aligned with each cathode electrode, and a second grid plate positioned between said first grid plate and said accelerating electrode and having a single aperture therethrough on said axis, and a second focussing means between said target and said first focussing means for imaging said common point on said target surface.

7. An electron gun for an electron discharge device, said electron gun comprising, means having electron emitting surface portions, a first control grid plate adjacent said electron emitting surface portions and having a plurality of apertures therethrough symmetrically disposed about an axis and overlying said electron emitting surface portions, a second control grid plate having a single aperture therethrough on said axis, and an apertured accelerating electrode coaxially positioned on said axis beyond said second control grid plate.

8. An electron gun for an electron discharge device, said electron gun comprising, means having electron emitting surface portions, a rst control grid plate adjacent said electron emitting surface portions and having a plurality of apertures therethrough overlying said electron emitting surface portions and symmetrically disposed about an axis, a second control grid plate having a single aperture therethrough on said axis, and an apertured accelerating electrode coaxially positioned on said axis beyond said second control grid plate, and means electrically connecting said first and second control grid plates.

9. An electron gun for an electron discharge device, said electron gun comprising, means having electron emitting surface portions, a tubular control grid electrode enclosing said electron emitting means and including a first control grid plate adjacent said electron emitting surface portions and having a plurality of apertures overlying said electron emitting surface portions, said apertures being symmetrically disposed about an axis, a tubular accelerating electrode coaxially arranged on said axis, and a second control grid plate between said first grid plate and said accelerating electrode, said second grid plate having a single aperture therethrough on said axis.

10. An electron gun for an electron discharge device, said electron gun comprising a plurality of cathode electrodes, each having an electron emitting surface, a tubular control grid electrode including a first control grid plate adjacent said cathode electrodes and having a plurality of apertures therethrough symmetrically disposed about an axis, means mounting said cathode electrodes within said tubular control grid electrode with one electron emitting surface opposite each grid plate aperture, and a second control grid plate having a single aperture therethrough on said axis.

11. An electron gun for an electron discharge device, said electron gun comprising a plurality of cathode electrodes, each having an electron emitting surface, a tubular control grid electrode including a rst control grid plate having a plurality of apertures therethrough symmetrically disposed about an axis, means mounting said cathode electrodes within said tubular control grid electrode with one electron emitting surface opposite each grid plate aperture, a tubular accelerating electrode coaxially arranged on said axis, and a second grid plate between said rst grid plate and said accelerating electrode, said second grid plate having a single aperture therethrough on said axis.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,099,749 Orth Nov. 23, 1937 2,177,366 Iams Oct. 24, 1939 2,457,175 Parker Dec. 28, 1948 2,556,824 Schade June 12, 1951 FOREIGN PATENTS Number Country Date 510,699 Great Britain Aug. 4, 1939 

